Ab initio calculations of oxygen nuclear quadrupole coupling constants and oxygen and silicon NMR shielding constants in molecules containing SiO bonds

Abstract

Ab initio coupled Hartree—Fock perturbation theory (CHFPT) calculations employing large gaussian basis sets have been used to evaluate the electric field gradient at O, q o, and the NMR shieldings at O and Si, σ o and σ Si, in the molecules SiO, SiO 2, Si 2O 2, H 2SiO, SiO −4 4, Si(OH) 4 and (H 3Si) 2O. Species containing Si bonded to three or fewer atoms have small NMR shieldings at both O and Si while those with four-coordinate Si have systematically larger O and Si shieldings. A significant positive correlation is observed between calculated O and Si NMR shieldings. Reduction of the SiO distance in SiO 4− 4 and H 3SiOSiH 3 gives a significant reduction in the magnitude of q o and a small increase in σ. Anisotropies in σ Si are large for two- and three-coordinate Si (200–900 ppm) but for (H 3Si) 2O the σ Si anisotropy is only ≈60 ppm. Anisotropies in σ o are generally larger than those in σ Si, with values larger than 200 ppm for both SiO −4 4 and (H 3Si) 2O. Values of q o for SiO 4− 4 and (H 3Si) 2O are in qualitative agreement with experimentally determined values for nesosilicates and SiO 2 polymorphs, respectively, but all the q o values appear to be exaggerated at the Hartree—Fock level. Also, q o values are not greatly different for the exotic species SiO, Si 2O 2, etc. compared to the typical silicate models SiO 4− 4 and (H 3Si) 2O. Calculated isotropic chemical shifts yield good values for the Si chemical shift differences of SiF 4, SiO 4− 4 and (H 3Si) 2O and for the O chemical shift difference of SiO 4− 4 and H 2O. For SiO 4− 4 the paramagnetic contribution to the Si shift, σ pSi is dominated by contributions from the t 2 symmetry SiO bonding orbital and σ pO is dominated by contributions from the t 2 symmetry O 2p non-bonding orbital.